1. Field of the Invention
The present invention relates generally to adaptive filters for canceling undesired signal components. The invention is particularly suitable for canceling multiple echoes resulting from the trans-hybrid coupling of a hybrid circuit.
2. Description of the Related Art
Adaptive filters are used in many applications for canceling undesired signal components. Echo cancelers are a typical application of the adaptive filter for canceling an echo resulting from the trans-hybrid coupling of a hybrid circuit with an echo replica derived from the input signal of the adaptive filter. The adaptive filter has a tapped-delay line and a tap-weight coefficient controller for producing a sum of tap signals weighted respectively by tap-weight coefficients. According to a known convergence algorithm such as the LMS (least mean square) algorithm, the tap-weight coefficients are upated by correlations between the tap signals and a residual error of a correction signal which is represented by the sum of the weighted tap signals. Fast convergence of the tap-weight coefficients are of primary concern for designing an adaptive filter. A typical fast convergence technique is described in "A Fast Convergence Algorithm for Sparse-Tap Adaptive FIR Filters for Cancellation of Multiple Echoes", Shigeki Ikeda et al., Proceeding of the 1994 International Symposium on Noise and Clutter Rejection in Radars and Imaging Sensors, 1994. 11, pages 265 to 270. According to this algoritm, the tapped-delay line is equally divided into tap groups and taps are sequentially selected on a per group basis and the tap signals of the selected taps are weighted by respective coefficients. The selected taps are moved from one group to another according to a hopping order and the dwell time of the selected group is determined from the coefficients.
However, when the adaptive filter encounters a low power input such as low speech activity, the growth of the tap-weight coefficients is not sufficient to reach optimum levels even though the tap position is optimum for high speech activity. If the tap position control is continued with low input power, the selected taps would be shifted. Therefore, when the input level is recovered, it would take a long time to converge the tap weight coefficients.